1. Analysing phytoplankton community data from 205 small, mainly oligotrophic Swedish lakes along a 13° latitudinal gradient, we found that the duration of the open-water season (DT > 0) was best related to phytoplankton species richness and biomass, probably because DT > 0 can be used as a proxy for nutrient concentrations, as well as for light and temperature conditions in lakes.
2. The relationships between DT > 0 and phytoplankton species richness and biomass were not linear but showed significant shifts, that is, richness suddenly doubled at DT > 0 = 170 days and phytoplankton biomass began to strongly increase at DT > 0 around 220 days.
3. The doubling in species richness corresponded to a change in the seasonal plankton succession from one to two peaks per year, and the shift in biomass corresponded to a shift in nutrient concentrations.
4. The geographical differences in richness and biomass shifts resulted in an exponential biodiversity–ecosystem functioning relationship along a DT > 0 gradient.
5. We conclude that phytoplankton species richness and biomass are driven by different environmental factors, resulting in a nonlinear biodiversity–ecosystem functioning relationship. The shape of the diversity–functioning relationship varies along with variations in environmental drivers, which is of highest importance for ongoing discussions about impacts of global change on biodiversity and ecosystem functioning.